In the publication ELECTRONICS LETTERS of 14 March 1985, Vol. 21, No. 6, pp. 249-251, a polarization-selective fused-fiber optical coupler which was manufactured from nonbirefringent single-mode fibers with matched cladding is described by M.S. Yataki, D.N. Payne, and M.P. Varnham under the title "All-Fibre Polarising Beamsplitter." It turned out that fused-fiber couplers with such single-mode fibers had to be drawn to a very long length to obtain the polarization-selective characteristic--approximately 10 cm to 30 cm. In the process, the fibers become very thin and are therefore very sensitive and difficult to handle. Furthermore, because of the high number of couplings occurring at this length, such a fused-fiber coupler is highly wavelength-selective, making manufacture for a specific wavelength more difficult, since the laser must then be selected and perhaps even stabilized in wavelength.
From the publication ELECTRONICS LETTERS of 9 May 1985, Vol. 21, No. 10, pp. 415 and 416, the article "Fibre-Optic Polarising Beam Splitter Employing Birefringent-Fibre Coupler" by I. Yokohama, K. Okamoto, and J. Noda reveals that it is possible to manufacture polarization-selective fused-fiber couplers with significantly shorter lengths using polarization-maintaining fibers. However, for this, only special types of fibers which are not standard in the trade may be used, such as with stress-applying parts (SAP) with matching refractive indexes, but which are very difficult to obtain and are very expensive.